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Considerable controversy regarding the effects of hypoxia on the pulmonary vasculature has arisen over the past few decades. The literature has been extensively reviewed by Fishman (1), and it is now generally accepted that hypoxia is a pulmonary vasoconstrictor. Much of the controversy has centered around the interpretation of a small elevation of pulmonary arterial pressure in the face of an increase in cardiac output related to the hypoxic stimulus. Some investigators calculated a small increase of pulmonary vascular resistance, whereas others could not confirm this observation. Most studies were performed in adult animals, or in adult humans, but more recently attention has been directed to hypoxic responses in the newborn animal, in view of the greater responsiveness of the pulmonary vasculature at this age. The relationship between the degree of pulmonary vascular response and the level of hypoxia has received only little attention, and usually a constrictor response to a single low oxygen gas inhalation has been reported. Recently Thilenius, Hoffer, Fitzgerald, and Perkins (2) attempted to relate pulmonary vascular resistance change to the level of inspired gas oxygen tension, but no obvious relationship could be demonstrated. Liljestrand (3) in 1958 suggested that the pulmonary vascular response to hypoxia may be related to the production of local Ho ion concentration changes in the lung. More recently Enson and as-

Response of the pulmonary vasculature to hypoxia and H+ ion concentration changes.